Slopes and Retaining Structures Under Seismic and Static Conditions

Retaining walls in Brazil have often been reinforced using nonwoven geotextiles instead of other reinforcement inclusions such as geogrids, woven geotextiles, and metallic reinforcements. This approach has several advantages such as ease of construction, expediency, and significant cost reduction. However, current lack of field data on the field performance of nonwoven geotextile-reinforced structures, mainly regarding displacements, has prevented their utilization in critical structures. Observation of the behavior of full-scale structures and a further characterization of the materials are needed to gain understanding of the behavior of nonwoven geotextile-reinforced soil structures. Accordingly, a series of nonwoven geotextile-reinforced soil structures was built and instrumented to analyze their behavior. This paper presents the results of a full-scale field monitoring program and associated finite element analyses conducted for a nonwoven geotextile-reinforced soil wall. The results show that current design methods provide conservative evaluation of the overall stability of the structure. The results of the numerical simulation indicate that FEM is a useful tool to predict the response of structures reinforced with geosynthetics. Monitoring results were found to be in agreement with those obtained in the numerical simulation.

The GlenEagles SRW wall is a 500 m long reinforced-earth structure, with a maximum height of 10.5 m and an average height of 7.5 m. Preliminary design was in strict accordance with conventional design principles (i.e., NCMA guidelines). The preliminary design was deemed overly expensive, and hence a literature review was conducted to identify research on design innovations that could be implemented to reduce the cost. It was found that the knowledge of the mechanics of reinforced-earth SRW behaviour has advanced significantly since the NCMA guidelines were formulated, and that the conventional design methods as presented in the NCMA and FHWA Design Manuals are in some areas overly conservative. The objective of this paper is to identify certain design guidelines that can be safely relaxed, leading to a more economical design. The paper describes the early design of the wall, the design innovations, and the construction of the wall. It also discusses various issues encountered before, during and after construction that had an important impact on quality and cost.

MSE walls with independent full-height facing panels (MSE/IFF walls) combine self-stable reinforced fill, full-height facing panels not attached to fill reinforcements, facing anchors that accommodate panel movement during construction, and a trench with flow fill and bracing for the temporary support of facing panels during construction. Colorado DOT built an MSE/IFF wall in 1996 along I-25 with two tiers of reinforcement lengths (8 ft long in the lower zone and 20 ft long in the upper zone). This wall system offers rapid and reduced construction work, an appearance similar to monolithic walls, and slender and lightweight facing panels since they carry only limited lateral pressure from fill. Since this MSE wall system is the first of its kind, it was considered experimental and a comprehensive instrumentation and monitoring program was performed. The objective of this research is to establish a workable and predefined MSE/IFF wall system and to refine design and construction methods used for the I-25 MSE/IFF walls. This paper reports the features, design, materials, construction, instrumentation, and measured performance of the I-25 MSE/IFF wall. Wall performance and design are assessed in terms of measured movements of facing panels and measured loads in fill reinforcements and facing anchors. The wall system performed as intended in the design. During construction, the wall facing was allowed to move outward sufficiently to mobilize tensile strains in the reinforcements and keep lateral earth pressure on facing panels low. After eight years, the I-25 wall shows no signs of distress or alignment problems.

Geosynthetic reinforcement has been successfully applied in slopes and retaining walls for three decades. Although reinforcement for foundation soils has been a subject of studies, no generally-accepted methods for design of reinforced foundation soils are available. Laboratory experiments reveal that the collapse mechanisms of foundation soils with strips of reinforcement have features similar to the mechanisms of unreinforced soils. A recent development will be presented where the kinematic approach of limit analysis is used to arrive at the bearing capacity of reinforced foundation soils. Two modes of foundation soil collapse are distinguished: the first one is associated with reinforcement slip, and the second one occurs due to tensile failure of the reinforcement. Limit analysis is applied to both cases, but only the second case yields the strict upper bound to the limit loads. The slip mode requires assumption of the stress distribution on the reinforcement, which relaxes the strict nature of the limit analysis method. The solution for uniform soils reinforced with geosynthetics was developed recently, and the method is now extended to two-layer foundation soils where a reinforced granular fill is overlying soft/weak deposits. The solution obtained for the increase in the foundation soil bearing capacity can be used as a guideline in design. A chart is presented to facilitate the use of the preliminary results. In addition to numerical assessment of the increase in bearing capacity, the analysis indicates the effective length of reinforcement and its placement (depth and spacing).

Recent earthquakes events in Japan, United States, Turkey, Iran, Italy have increase the interest to the analysis of soil structures subjected to seismic loads. The common procedure used to design slopes is based on limit equilibrium methods. The method is used to analyse the stability of slopes with and without reinforcements. The performance of reinforced soil slopes were investigated in this paper by a number of shaking table tests carried out in the University of Catania shaking table facilities. The required length of reinforcements was calculated using the relationship of Jewell as modified and proposed by Cascone, Maugeri, Motta for the seismic design. All reinforced slopes models were built with the inclination respect the horizontal of 70° and height of 0.35m. The soil used in the tests is a typical dry silica sand of the East Coast of Sicily. The reinforcement, chosen to obtain a good scale factors with the model, is a bi-directional polypropylene geogrid. Each tests is subjected to a uniform surcharge obtained by small block elements with the purpose to apply a load value of 1 kPa. The models were subjected to a sinusoidal progressively increasing acceleration and to the E-W component of Catania earthquake of 1990. Moreover the shaking table tests were performed to investigate the effects of the reinforcements vertical spacing. Model deformations were found to be depending on the reinforcements spacing and on the level acceleration of the input motion. Two-wedge failure mechanism is a dominant one as showed by the failure surfaces observed.

Utilizing waste materials as an alternative to using virgin construction materials, made from nonrenewable resources, in civil engineering applications is currently one of the most important environmental issues. The unique properties of tire chips such as strength, high frictional resistance and flexibility can be exploited in a beneficial manner in geotechnical applications. This research aims to investigate the benefits of using tire chips as lightweight inclusion to improve the bearing capacity and control the settlement of sandy soil. Forty loading laboratory tests were carried out on a strip-footing model of 50 mm width supported by sand-tire chips mixture. The study was carried out on sand with relative densities of 50%, 75%, and 90%. The thickness of sand layer containing the inclusions underneath the footing was taken equal to 1, 2, 3, and 5 times the strip footing width. The mixture overlies pure sand layer. The mixture was formed at the same relative density as the underlying sand. The percentage of tire chips to sand was taken as 5%, 10%, and 15% by weight. The study revealed that at the same loading level the settlement of strip footing on sand-tire chips mixture was about 30% less than in the case of pure sand. The initial modulus of elasticity increased to a value varied from 2 to 9 times its value in the case of pure sand at the same relative density. Most importantly, the ultimate bearing capacity of sandy soil noticeably increased up to 7 times its value in the case of pure sand. The conclusions of this paper support the use of tire-chips as an environmentally acceptable lightweight inclusion in soil improvement applications.

Reinforced soil slopes using tire chips from the waste tires have gained importance in recent construction practices. The objective of the present study is to evaluate the engineering properties of Powai soil, waste rubber tire chips and soil tire chips mixture relevant to geosynthetic-reinforced earthwork. Tests were conducted to obtains hear strength parameters of soil and soil-tire chips mixture. The centrifuge modeling was carried out to study the slope stability for varying D/H ratio (where D = depth of the foundation soil and H = height of the slope) for un-reinforced and reinforced soil slopes. The analysis of the centrifuge modeling results was done using Reinforced Soil Slopes (RSS) Program to obtain the factor of safety of the slopes. This study shows an increase in the factor of safety with increase in the percentage of the tire chips (2.5 % to 10 %)

Geosynthetic pack piles were manufactured by using polyester (PET) and Basalt woven mats and their properties in weak ground reinforcement applications were examined. The purpose of these packs is to facilitate and act as flexible forms for the construction of concrete piles. Experimental data from tests, geosynthetic pack piles were evaluated and compared. Index tests and large-scale laboratory pullout tests were performed. Basalt woven mats showed better performance as compared to PET mats in terms of tensile properties and chemical resistance. On the other hand, PET mats showed excellent seam strength and pullout properties than Basalt woven mat. Especially, 10T/m PET mat showed the most stable pullout property among tested geosynthetics pack piles.

A numerical model has been developed using the program FLAC to simulate the response of geosynthetic reinforced soil retaining walls constructed with a modular concrete block (segmental) facing. The model has been verified by comparing numerical results with physical measurements taken from three full-scale (i.e. 3.6 m-high) reinforced soil walls that were constructed in a controlled in-door test facility. The walls had different reinforcement configurations, but were otherwise identical. The predicted responses for all three walls with respect to facing displacements, footing loads, foundation contact pressure and reinforcement strains were in satisfactory agreement with the measured data. The numerical model provides a validated tool that can be used to extend the database of full-scale experimental wall test results through parametric analyses that investigate the influence of wall type, geometry, component properties and other field conditions on wall performance during and at the end of construction.

The paper provides a design/analysis method for reinforced soil retaining walls where the extent of the reinforced fill zone is constrained by the presence of a rock or heavily overconsolidated soil outcrop. The effect of the geometry of the constrained reinforced fill zone on the magnitude of horizontal stresses acting on the wall face is presented. For internal stability residual tensile stresses in the reinforcements at the rear of the constrained reinforced fill zone have to be dissipated either by extending the length of the reinforcements within the reinforced fill zone, or by connecting the reinforcements to low capacity anchors or nails fixed into the rigid zone beyond. A reinforced segmental block retaining wall case study with a constrained reinforced fill zone is presented where the solution for internal stability was to connect the geogrid reinforcements to low capacity anchors fixed into a quartzite rigid zone beyond the constrained reinforced fill zone.

This paper describes a technique to calculate the system probability of failure (or system reliability) of MSE walls. The system probability of failure is defined as the complement of the sum of the probabilities of failure corresponding to internal failure and external failure (sliding, overturning, and bearing capacity failure). The reliability is the complement of the system probability of failure. Probability of internal failure is obtained from analysis of MSE walls using design charts. Probabilities for each external failure mode are calculated using a mean factor of safety computed with conventional design equations and mean values for the soil properties as input, and a standard deviation of the factor of safety computed using an empirical equation. The empirical equation relates the standard deviation of the factor of safety for each external failure mode to uncertainty in the design parameters. Analyses of three existing walls suggest that conventionally designed MSE walls have a system probability of failure ≤ 0.0003, which is one to two orders of magnitude lower than the failure rate for similar geotechnical structures.

This paper presents two case histories of segmental block, mechanically stabilized earth (MSE) retaining walls. Sites for the two walls are located in the Valley and Ridge physiographic province of southwestern Virginia, and eastern Tennessee. Both walls were greater than 7.6 meters (25 feet) high and were constructed using geosynthetic reinforcement within a clayey soil backfill. One of the walls failed; the other experienced large deformation, but remains in service. The paper presents the results of the forensic investigations and offers guidance for future designs. Factors contributing to the observed failures include not only the technical, but also professional issues. Technical issues include (but are not limited to) the engineering behavior of clayey/poorly draining soils within a reinforced soil structure. Professional issues include the adequacy or completeness of design calculations and of the resulting plans and specifications.

A bend in a pipeline with internal pressure will generate thrust externally. For a buried pipe, this thrust is supported by the passive earth pressure through the backfill at the bend, which is typically supported by the concrete blocks. However, the use of this heavy concrete block in the soft ground is not desirable, because the local settlement of pipeline will occur. In this paper, new thrust protecting methods using vertically installed geogrid were proposed. Lateral loading tests were conducted using a pipe model having 90 mm diameter and that was attached geosynthetics vertically. Similar tests were conducted using a square model pipe assumed concrete block for direct comparison. Two types of dense condition and loose condition were used as backfill. As the results, it was found that these new methods were very effective as a thrust protecting method for lateral forces due to internal pressure.

The behavior during construction of a multi-step type Multi-Anchored Reinforced Soil Retaining Wall constructed on relatively soft foundation ground was carefully observed by means of various sensors. The aim of this study is to provide valuable information for the verification of the design concept of the multi-anchored reinforced retaining wall through the analysis of the observed behavior of the reinforced retaining wall. In this paper, first the mechanism of reinforcement and the design concept in the multiple-anchored reinforced soil method are briefly introduced. Next, the characteristic design of the three-step reinforced retaining wall is explained regarding the advantages of mechanical stability and cost performance. The observed behavior of the reinforced retaining wall is analyzed and the applicability of the design concept described in the available design manual for Multi-Anchored Reinforced Soil Method is discussed. According to the result of the measurement, sufficient stability of the multi-step reinforced retaining wall was recognized. The safety factor of the sliding arc considered reinforced effect was calculated as 1.5 based on the measurment.

The study investigates possible effects of stress path on a series of consolidated-drained triaxial compression and extension tests. Test results of unreinforced and reinforced residual soil simulating the different loading path similar to what is expected in the field. The testing program concentrated on the study of normally consolidated on 100 mm diameter and 200 mm high specimens. Testing was carried out at consolidating pressures ranging between 0.10–0.60 MPa using GDS computer control triaxial apparatus. The Cam-Clay model parameters for unreinforced and reinforced residual soil were determined based on isotropically consolidation and drained shear test results. Computations were performed using the model constant and finite element software SIGMA/W to predict the stress-strain response for different consolidation pressure and stress path tests. Predicted results were compared with measured response to assess the validity of the model parameters. The model prediction was found to yield acceptable results and is quite effective in demonstrating the effects of stress paths, effective stresses and strains of reinforced soil composites.

A novel use of a composite liner consisting of a fluorinated high-density polyethylene (HDPE) geomembrane and needle-punched geosynthetic clay liner (GCL) to contain a hydrocarbon spill in the Arctic has been described in a previous paper. This paper reports the results of laboratory tests conducted to assess the effect of exposure to cold climates and hydrocarbons on the performance of the GCL and HDPE geomembrane with time. Results are reported for tests conducted to examine the effect of freeze-thaw conditions on the short-term low gradient and the long-term high gradient hydraulic conductivity of the unfrozen GCL with respect to Arctic diesel (jet fuel). Additional studies examine the hydraulic conductivity of the frozen GCLs. The behavior of the geomembrane is also examined with particular reference to the effects of contact with jet fuel on the chemical and mechanical properties of the geomembrane. The implications of these extreme conditions over the period when the composite liner can be expected to contain hydrocarbons are discussed.

Normal soil reinforced walls as a sole solution for marine structures may not be so effective in harsher marine environment due to severe hydrodynamic forces due to waves, tides and currents. It is more effective if used in combination with other geosystems. Geotubes and Geocontainers can be used to construct marine structures by itself or as a core of other geosystems. The stability of these systems can be further enhanced by using stacked geotubes, proper strapping system to keep number of tubes as single unit, adequate apron on headward side and suitable anchorage system. The static stability of such type of geosystem has been explored by 2-D analytical modeling and substantiated by 3-D finite element modeling using FLAC. Closed form and approximate solutions for the cross-sectional shape and circumferential tension for stacked tubes were obtained for partial or full submergence, resting on deformable foundations, subjected to asymmetric loadings (as in dikes). Dynamic stability of geotubes against current and wave attack has also been investigated. A two-dimensional equilibrium and vibration analysis of a freestanding geocontainer is being done. Air and water are the two internal materials investigated. Three foundation variations are considered: rigid, Winkler, and Pasternak. Mathematica 4.2 was employed to solve the nonlinear equilibrium and dynamic equations, incorporating boundary conditions by use of a shooting method. The additional features of viscous damping and added mass of the water were applied to the rigid foundation study of the vibrations about the freestanding equilibrium configuration. Results from the equilibrium and dynamic analysis include circumferential tension, contact length, equilibrium and vibration shapes, tube settlement, and natural frequencies. Natural frequencies for the first four mode shapes were computed.

Geosynthetics are used in the design and construction of various mining facilities for process solution containment (heap leach pads, solution ponds and tailings impoundments), foundation stabilization and support, and reclamation. Due to the nature and location of mining projects, the performance envelope of geosynthetic materials is often pushed beyond the limits of typical design procedures, testing, and construction methods. Common issues addressed in mining applications include: (1) Geomembrane liner and plastic pipe performance under very high loads (in excess of 3 Megapascals [MPa]); (2) Solution containment liner systems founded on compressible fills; (3) Liner construction under harsh environments; and (4) Foundation stabilization over historic mine workings. To cope with these harsh conditions, innovative testing, design and construction approaches have been developed. A non-standard laboratory liner-load testing frame is used to define the performance of the liner under high loads. This test frame is also used to evaluate the performance of the over- and under-liner materials. High-load interface shear tests are also used to assess stability of the liner system. The performance of plastic pipe under high loads is evaluated using a modified analytical method that accounts for arching effects in the over-liner material and acceptance of high deformation in the pipe cross-section. Methods to evaluate liner foundation deformation are also discussed with an emphasis on construction/stabilization. In addition, considerations for liner deployment and placement in harsh climatic conditions are discussed.

Geomembranes are one of the most commonly used geosynthetics in landfill liner systems. They retain the leachate produced by the waste and prevent leakage. Geomembranes may experience harsh environmental conditions such as extreme temperatures or earthquake loading. Earthquake loading can be an extreme loading case for landfills located in seismic regions. This study, based on dynamic centrifuge testing, investigates the effects of simulated earthquake loading on the tension experienced by the geomembrane on a landfill slope. The landfill modeled in the dynamic centrifuge test was a municipal solid waste (MSW) landfill cell with a single geomembrane-clay liner system (45° side slope and 10 m slope length). The paper shows that moderate earthquake loading (base acceleration between 0.1 g to 0.2g) can result in transient increases of around 20% in geomembrane tension, with permanent tension increases of around 5%.

This paper presents the results of a study on the use of geosynthetic reinforced embankments for the protection of buried pressurised pipes by means of laboratory tests. The expansion of a pipe inside reinforced and unreinforced sand was simulated in models studies in the laboratory using a large rigid steel tank. Different types and configurations of geosynthetics were tested including woven geotextiles, geogrids and metallic reinforcements. The scale of the models in the laboratory was 1:5 and the series of the tests involved a continuous expansion of the pipe walls (thin rubber membrane) until failure of the soil mass was achieved. The results obtained showed a significant influence of the presence of the reinforcement on the pressure in the pipe, depending on reinforcement type and arrangement, and the potential use of reinforced embankments to protect pressurized pipes.

The national movement to develop a mechanistic-empirical design guide for pavements requires that the fundamental material properties for all components of the design be quantified. When geosynthetics are used to reinforce the base course layers of flexible pavements, one of the two main design parameters is the interaction between the geosynthetic and the surrounding aggregates. Interaction at this interface can be quantified in terms of a stiffness parameter, G_i, the resilient interface shear modulus. The most relevant interaction tests use cyclic loads like those experienced in transportation applications. Currently, however, there is no standard test to quantify soil/geosynthetic interaction using cyclic loads. This research effort modified the standard pullout test protocol to resemble the resilient modulus tests for unbound aggregates which utilizes cyclic loads at various levels of normal confinement. The resilient modulus for unbound aggregates (M_R) closely resembles G_i, since they are both simultaneously dependent on shear load and confinement. Overall, the results from the cyclic pullout tests conducted on six geosynthetics showed that cyclic pullout testing has great potential for describing a stress dependent interface shear modulus. A three-parameter, log-log equation developed in the NCHRP Project 1-28a (NCHRP, 2000) was used to predict G_i. Correlations between predicted and measured values were somewhat erratic. Additional research is planned to improve the test equipment and establish specific test protocols.

The interface resistance between the geogrid reinforcements and cohesive backfills are dependent upon the failure mechanism involved, the properties of the backfill, the test conditions and the geometry and stiffness of the geogrids. In the current study, the results of field and laboratory pullout tests, conducted by Farrag and Morvant, were compared to the pullout resistances predicted using the relationships readily available for calculating the frictional and bearing resistances of the reinforcements. The calculation of the bearing resistances was based on the relation recommended by Peterson and Anderson, whereas the frictional resistances were calculated based on the effective (solid) frictional areas of the geogrids with two interface (shear strength) factors of (0.8 and 1.0). The field pullout tests were conducted in a 20 ft (6 m) high wall constructed at the Pavement Research Facility (PRF) of the Louisiana Transportation Research Center (LTRC), while the laboratory tests were conducted in a test box in the geosynthetic engineering laboratory of the LTRC. Four geogrids were used in this study: Stratagrid-500, UX-750, UX-1500, and UX-1700. The tests were conducted at different levels of stress and different reinforcement lengths. Based on the comparison of the total predicted and the laboratory tests results, the relationship recommended by Peterson and Anderson was found to conveniently predict the contributions of the bearing resistances of the transverse members of the reinforcement. However, the same relationship was found to have more significant errors in predicting the bearing contributions of the pullout strips in the field. The contributions of the bearing resistances to the pullout capacities of geogrids were found to vary from 8% to 40%, depending on the types, geometry and stiffness of the geogrid.

The creep behavior of a high density polyethylene (HDPE) geogrids used for soil reinforcement applications is evaluated using two accelerated creep tests; time-temperature superposition (TTS) and stepped isothermal method (SIM). TTS is a well-accepted acceleration method to evaluate viscoelastic behavior of polymeric materials, while SIM was developed in the last ten years mainly to shorten testing time and utilize a single test specimen to minimize material variability. The applicability of the SIM test is well demonstrated, particularly for polyethylene- terephthelate (PET) geogrids. However, for HDPE geogrids, the application of the SIM has not been well established. In this paper, the creep behaviors of HDPE geogrids are evaluated using both TTS and SIM creep procedures. For the SIM procedure, four test parameters were investigated. The study confirmed that ASTM recommended values for temperature steps of 7°C and a dwell time of 10,000 seconds are suitable for HDPE geogrids. At applied loads of 10 and 20%, only the primary creep state was measured, while secondary creep state appeared at applied loads of 30 and 40%. In addition, test specimens with and without transverse junctions exhibited significant difference in creep behavior. The test specimen with junction (i.e., two longitudinally aligned ribs) displayed greater creep compliance than one without a transverse junction. Based on the same duration time of 10,000 seconds, the resulting creep behavior from SIM and TTS is very similar at 20 and 30% UTS. However, the activation energies are much higher than those long-term creep tests.

A series of direct shear experiments were performed to examine the interface friction between fiber reinforced soils and a geomembrane. The soil used in this study classified as a clayey sand (SC) according to the USCS. The tests were performed in a standard direct shear device. The soil was compacted and tested at its optimum moisture content with varying dosages of fibers. Dosage rates of 0%, 0.1%, 0.2%, and 0.3% by dry weight were used. This work was performed by an undergraduate for credit as part of a new undergraduate research class at Villanova. The results indicate that to maximize shear strength, there is an optimum dosage rate of 0.2%. As this work was a preliminary study, there is still much research to be completed in this area. For example, additional tests should be conducted in large, 30.5 cm direct shear devices and additional tests on textured geomembranes and other soils are recommended.

Rainfall-induced landslides are common in Puerto Rico (PR). The presence of steep slopes in mountainous terrain, coupled with weathered soils and intense rainfall, leads to severe slope-stability problems throughout the island. Episodic triggering events such as hurricanes and earthquakes further exacerbate these problems. All physiographic provinces of the island have experienced landslides. The stability of natural and man-made slopes is a serious concern for government authorities and the civil engineering community in Puerto Rico. This paper presents an overview of the rainfall induced landslide problem in PR, a summary of literature published on this subject, and proposes a rainfall intensity landslide threshold based on landslide events data from 1959 to 2003. This threshold can be used as part of a potential landslide warning system.

This paper deals with slope stability considerations associated with an approximately sixty foot (20 m) high cut excavation through various rock formations, for a solid waste sanitary landfill. During rock quarrying operations to prepare site areas for the development of the sanitary landfill, limited sectors of the top of slope experienced outward and/or downward movement and talus tension cracks developed within the upperlying soil layer near the apex of the site. The results of extensive field observations, evaluation of the rock cut operations, field and laboratory test studies and knowledge of the geological characteristics of the various rock formations were used to determine the probable reasons for the rock movement. Redesigned rock slope configurations and management plans to control flow of water into the landfill deposits were developed for safe execution of the landfill operations.

It has been proven that the groundwater plays a fateful role in the safety factors of slopes. However, in the design of the slope stability in the Three Gorges Reservoir Zone (TGRZ), the methods adopted for determining the ground water table and hydrodynamic forces during reservoir drawdown are mostly empirical and vary from one project to another. To improve the methods, a simplified formula of the phreatic-line in landslide mass was proposed during reservoir drawdown in this article, based on the Boussinesq's differential equation of unsteady-seepage, and deduced by Laplace's transform. The formula is a polynomial about λ which is a comprehensive parameter determined by the following factors: Coefficient of permeability, specific yield, descendant velocity of drawdown, thickness of aquifer and height of drawdown. By analyzing the formula, the authors found that the larger λ, the slower the ground water table drawdown, and the more unstable the slope. To reduce the error caused by one-dimensional instead of two-dimensional, the formula was modified on the basis of numerical analysis. In the end, the amended formula was proved by a large-scale model test. The test shows that the value obtained is much close to that by the amended formula, with the error within 5%. At tip of slope, the test value is a bit greater than that by the amended formula, and with the adverse condition at the middle and top of the slope.

This paper introduces a series of analyses of transient groundwater flows in a soil slope under reservoir water level fluctuations and analyses of the stability of the slope. In the seepage analyses, groundwater regimes during reservoir filling and reservoir water level drawdown are studied. The soil-water characteristic curve of the slope soil is estimated from the grain-size distribution of a typical landslide soil and the permeability function is estimated based on in-situ permeability test results and the estimated soil-water characteristic curve. The boundary condition of the slope surface in the water level fluctuation zone is set as a known hydraulic-head function of time. Delayed groundwater response in the slope is found when the reservoir water level fluctuates. The stability of the slope during reservoir water level rising and water level drawdown is also analyzed incorporating the pore water pressures calculated from the transient seepage analyses. These analyses provide some basis for stabilization of similar slopes subject to reservoir water fluctuations.

The study of multiphase flow in porous media, (e.g., air and water flow in unsaturated soils), is relevant to many areas such as contaminant transport and remediation, oil extraction, concrete materials behavior, geothermal reservoir engineering, and landslide prevention. Air-phase flow is the primary concern of this paper. The relevance and concepts of soil-air characteristic curve are introduced. A procedure is proposed to establish the relationship between the air-phase permeability and matric suction. The Klinkenberg effect, which involves an enhancement of air-phase permeability through slippage of air molecules along boundaries of air-filled pores, is also considered in the air permeability function. Two case studies are used to validate the proposed procedure based on experimental data reported in the literature. Reasonable agreements are observed between the derived air permeability equations and test results in both cases.

During the spring of 2003, the downstream slope of an embankment dam began to move during an extreme flooding event. The current study was implemented to evaluate the slope stability of the embankment and as a case-study to compare the limit equilibrium and continuum modeling methods. Initially, stability analyses were performed using conservative shear strength of soils estimated from both boring log data and an infinite slope back analysis. The Mohr-Coulomb failure criterion was employed in the computer simulations. The computer program UTEXAS4 and Spencer method of solution was used to evaluate the limit equilibrium factors of safety and the critical failure surface. The computer program FLAC4.0 was used to evaluate the stress/strain behavior of the slope and determine factors of safety using the strength reduction method. Both these limit equilibrium and continuum models yielded slip surfaces that were close in geometry and location to the actual slip surface observed in the field. The factor of safety for the pre-failure embankment geometry was lower utilizing the limit equilibrium approach than that estimated by the continuum approach; however, the continuum approach appeared to yield more accurate results when using the best estimate of the shear strength. Back-analyses performed based on the observed failure surface in the field after flooding event provided estimates of shear strength of the embankment soil. The back calculated shear strength utilizing the continuum model fell within acceptable ranges as determined from the boring log data, with the continuum model yielding the more conservative (lower) back calculated shear strength. Back calculated shear strength using the limit equilibrium method was higher and its use may result in unconservative designs.

Reservoir slope instability is a geological hazard induced by filling a reservoir and is generally believed to be caused mainly by unfavorable changes in the hydrodynamic condition in the slope during reservoir filling and/or operation. Although many cases of reservoir slope instability have been reported, very few field instrumentation data exist describing how the hydrodynamic condition changes and relates to landslide reactivation. Taking the opportunity of filling the Three Gorges Reservoir, a monitoring system was implemented to monitor the hydrodynamic and deformation response of a preexisting landslide, the Xietan landslide in Zigui County, Hubei Province, China. The monitoring items included reservoir level, water level in boreholes, pore water pressure, rainfall precipitation and deformation of borehole using inclinometers. The results indicate that there exist two phreatic surfaces in the slope before reservoir filling, the upper one is at around the top surface of the slip band and the lower one is in the bedrock. The less permeable slip band and sliding-disturbed zone constitute the relatively impermeable layer between the two surfaces. The stabilization effect of the reverse seepage during reservoir filling is temporary and limited. Reservoir filling is the main cause of reactivation.

Instability behavior of the levee on soft foundation at the end of construction and during rapid drawdown, respectively, is investigated by centrifuge tests. Soft ground is made from the remolded clay cakes by using muddy clay from in-situ soft ground of Huai-He River's levee of a flood channel to sea. The factor of safety against sliding is acquired by overloading the model levee to failure by increasing the centrifugal acceleration, and the surface of sliding is determined from the map of vectors of movement on slope cross-section. It is found that concerning the instability behavior of levee on soft foundation at the end of construction, the sliding surface is a deep seated one, which passes through the soft ground; on the contrary, it is seen that concerning the mechanism of instability during sudden drawdown of floodwater, a general failure of sliding is preceded with a local failure of slope instability in embankment slope, whose slip surface passes through the toe of slope. These test results show that the stability of levee on soft ground is mainly governed by the bearing capacity of the soft soil layer and is influenced, to a certain degree, by action due to sudden drawdown.